Photoplethysmography (PPG) involves obtaining a volumetric measurement of tissue and/or vessels in response to blood flow. One current non-invasive method is known for measuring the oxygen saturation of blood using pulse oximeters. With each cardiac cycle the heart pumps blood through vessels creating a pressure pulse. This pressure pulse distends the arteries and arterioles in the subcutaneous tissue. When the heart pumps blood to the body and the lungs during systole, the amount of blood that reaches the capillaries in the skin surface increases, resulting in more light absorption. The blood then travels back to the heart through the venous network, leading to a decrease of blood volume in the capillaries and less light absorption. The pressure pulse may also be detected in veins, e.g., from the venous plexus. In PPG, a light source is directed at skin tissue and reflected light is detected by a photodetector to generate a PPG signal. The change in blood volume during a pressure pulse affects the amount of light reflected to a photodetector. As such, the pressure pulse appears as a peak in the PPG signal. The measured PPG waveform therefore comprises a pulsatile (often called “AC”) physiological waveform that reflects cardiac synchronous changes in the blood volume with each heartbeat, which is superimposed on a much larger slowly varying quasi-static (“DC”) baseline.
Heretofore, photoplethysmography was used primarily in pulse oximeters to detect a heart rate and oxygen saturation levels. In pulse oximetry, the subject's skin at a ‘measurement location’ is illuminated with two distinct wavelengths of light and the relative absorbance at each of the wavelengths is determined. For example, a wavelength in the visible red spectrum (for example, at 660 nm) has an extinction coefficient of hemoglobin that exceeds the extinction coefficient of oxihemoglobin. At a wavelength in the near infrared spectrum (for example, at 940 nm), the extinction coefficient of oxihemoglobin exceeds the extinction coefficient of hemoglobin. The pulse oximeter filters the absorbance of the pulsatile fraction of the blood, i.e. that due to arterial blood (AC components), from the constant absorbance by nonpulsatile venous or capillary blood and other tissue pigments (DC components), to eliminate the effect of tissue absorbance to measure the oxygen saturation of arterial blood. PPG techniques have also been described for measuring other blood constituents, such as in U.S. Pat. No. 9,642,578, entitled, “System and Method for Health Monitoring using a Non-Invasive, Multiband Biosensor,” issued on May 9, 2017, and incorporated by reference herein in its entirety. In general, when detecting such blood constituents, motion artifacts in the PPG signals are ignored or filtered from the signals.
In one or more embodiments described herein, the PPG signal is used to non-invasively detect movement and/or neural stimulation of a body part of a user.